Do you have a scale? Anything 13oz or less goes First Class which is under $4. After that, stuffing as much as you can into a Padded Flat Rate Envelope for $5.10 is your best bet. From there, Priority is almost always within $1 of the Parcel Post cost, but takes 2-3 days vs 2 weeks...it's worth the $1! Flat Rate boxes are your friend! The supplies are free from USPS.

When you ship through Paypal, the rates are a bit cheaper, plus you get free DC (which I'm sure you know is mandatory here)